CN1175315A - Capacitance-based proximity sensors with interference rejection apparatus and method - Google Patents

Abstract

The invention relates to apparatus and method for a capacitance-based proximity sensor with interference rejection. A pair of electrode arrays (90) establishes a capacitance on a touch detection pad (12), and the capacitance varies with movement of a conductive object near the pad (12). The capacitance variations are measured synchronously with a reference frequency signal to thus provide a measure of the position of the object. Electrical interference is rejected by producing a reference frequency signal which is not coherent whith the interference.

Description

Capacitive based proximity sensor utilizing interference rejection apparatus and method

The present invention relates generally to an apparatus and method for use as a touch-sensitive input device, and more particularly to a capacitive-based apparatus and method for touch detection in which electrical interference from the detection system is effectively excluded.

There are many devices and systems in the prior art that utilize the sense of touch to input data to a data processor. Such devices are used in place of conventional pointing devices such as a mouse or stylus, providing data input by fingers resting on a pad or on a display device. These devices detect finger position through a capacitive touch pad, where a scan signal is applied to the pad, and detect changes in capacitance caused by a finger touching or approaching said pad. By detecting the position of the finger at successive moments, the motion of the finger can be detected. This detection device can be used to control a computer screen cursor. More generally, it can provide information corresponding to finger movements, postures, positions, writing, signing and drawing movements to various electrical devices.

In US Patent No. 4,698,461 to Meadows et al., a touch surface covered with a layer of constant resistivity is disclosed. A screen scan signal is applied to energize selected edges of the touch surface such that an AC voltage gradient is established across the surface of the screen. When the surface is touched, a touch current flows from each actuated edge through the resistive surface, which then reaches the user's finger (either capacitively or conductively), passes through the body, and finally connects through body capacitance and ground potential. Different scan sequences and voltage forms were applied across the edges and the current was measured in each case. By measuring these currents, the location of the touch can be determined. In particular, the physical parameter representative of the touch location is the resistance from the edge to the touch point on the surface. This resistance will change as the touch point approaches or moves away from each edge. For such systems, the term "capacitive touchpad" may be a misnomer, since the capacitance involved is a parameter used to couple currents from a surface touch point through a person's finger, rather than representing finger position. A disadvantage of this invention is that accurate touch location measurement depends on the uniform resistivity of the surface. Fabricating such surfaces with uniform electrical resistance is difficult and expensive, and requires specialized fabrication methods and equipment.

The screen of the '461 Meadows patent also includes a circuit for "nulling" or biasing to zero the touch current present when the screen is not being touched. Returning to zero can be carried out when operating the screen and thus enables a more accurate determination of touches which generate relatively weak signals, for example with a gloved finger. The screen of the '461 Meadows patent also includes a circuit for automatically shifting the frequency of the screen scanning signal away from the spectrum of spurious signals, such as those generated by cathode ray tube transformers, which may be present in the environment. The screen tries to avoid interference from spurious signals, which can occur if the scanning frequency is nearly equal to the frequency of the spurious signals. The microcontroller determines whether to move the scan frequency by monitoring the number of touch current zeroing adjustments required as described above. The only method described for generating the frequency control signal is based on this zeroing adjustment.

US Patent Nos. 4,922,061 to Meadows et al. and 461 to Meadows are similar in that the touch screen determines touch location based on changes in resistance rather than changes in capacitance. This is especially evident from Figure 2, where the resistance from the edge to the touch point is expressed as Kx times Rx, where Kx corresponds to the distance represented by 76A, the device uses a frequency that varies substantially in a random manner as Measure signals, reducing susceptibility to spurious sources from the electromagnetic spectrum.

US Patent No. 4,700,022 to Salvador describes an array of conductive strips for detection, each conductive strip positioned between resistive emitting strips. The finger actually makes electrical contact from the transmitter strip to the detection strip. The location of the touch is determined by the change in resistance in the strip of finger contact (as in the aforementioned Meadows '461 and '061 patents). Takes the average over some number of simultaneous samples. There is a design formula used to choose the sampling frequency so that it is not a multiple of the least desirable intended interfering signal. There is no prompting for sampling frequency adjustments or automatic adaptation of the sampling frequency.

In US Patent No. 5,305,017 to Gerpheide, the touch location is determined by a true change in capacitance rather than by a change in resistance. Among them, many electrode strips are used to form effective (Virtual) electrodes. This method does not require a uniform resistor covering the display. However, such capacitance-based detection devices may be disturbed by background electrical noise around them, which is coupled onto the detection electrodes and interferes with position detection. These spurious signals create potentially glitchy interference, interfering with the detection of finger positioning. The device operator can even act as an antenna for electrical interference, causing spurious charge injection or elimination to the detection electrodes.

Therefore, a touch detection system is needed, which has the following characteristics:

(1) The touch position can be determined without resistance variation, thereby eliminating the need for high costs for uniform resistance during manufacture;

(2) Touch location is measured independently of electrode resistance or its wiring, thereby expanding the range of materials and methods available for fabrication; and

(3) Regardless of the frequency of the electrical interference signal, the electrical interference signal can be eliminated from the detection system without an expensive zero return device.

The present invention employs a touch location device that has a true capacitive change by using an insulated electrode array to form the active electrodes. The measurement of the capacitance change is carried out by means of a method independent of the electrode resistance, so that the electrode resistance parameter does not have to be taken into account during the manufacturing process. Regardless of the frequency of electrical interference, it can be eliminated, thereby providing a reliable detection signal.

An illustrative embodiment of the invention includes an array of electrodes to produce a capacitance that varies with the motion of an object (e.g., a finger, other body part, a conductive stylus, etc.) in the vicinity of the array for a synchronous capacitance measurement section for measuring changes in capacitance, said measurement being synchronized with a reference frequency signal, and a reference frequency signal generator for generating a reference frequency signal that does not introduce electrical interference that interferes with the capacitance measurement , thus not interfering with position fixes.

Interference rejection is performed by generating a reference frequency signal whose frequency is different from the interference frequency. In addition, the reference signal is generated with a random frequency so that it will not be coherent with the interference frequency, thus effectively eliminating the interference.

Fig. 1 is the block diagram of the capacitance change position measuring device realized according to the principle of the present invention;

Figure 2A is a plan view of an illustrative embodiment of the electrode array shown in Figure 1;

Figure 2B is a side cross-sectional view of an illustrative embodiment of the electrode array of Figure 2A;

3A is a side cross-sectional view of another embodiment of the electrode array of FIG. 1;

Figure 3B is a plan view of the electrode array of Figure 3A;

Fig. 4 is a schematic diagram of an embodiment of the synchronous electrode capacitance measuring device of Fig. 1;

Fig. 5 is a schematic diagram of another embodiment of the synchronous electrode capacitance measuring device of Fig. 1;

6A-6D are circuit diagrams of another embodiment of the capacitance measuring portion shown in FIGS. 4 and 5;

Figure 7 is a block diagram of one embodiment of the reference frequency generator shown in Figure 1; and

FIG. 8 is a block diagram of another embodiment of the reference frequency generator shown in FIG. 1 .

Figure 1 shows a capacitive change finger (or other conductive body part or conductive non-body part) position detection system 10 implemented in accordance with the present invention. Electrode array 12 includes multiple layers of conductive electrode strips. The electrodes and the wires connecting the electrodes to the device can have substantial electrical resistance, allowing the use of a variety of materials and processes for fabrication. The electrodes are electrically insulated from each other. There is a mutual capacitance between every two electrodes and a stray capacitance between each electrode and ground. A finger positioned near the array changes the value of these mutual and stray capacitances. The extent to which these capacitance values change depends on the position of the finger relative to the electrodes. In general, the degree of change increases the closer the finger is to the electrode concerned.

A synchronized electrode capacitance measurement unit 14 is connected to the electrode array 12 and determines selected mutual capacitance values and/or stray capacitance values associated with the electrodes. In order to reduce interference, a large number of measurements are performed by the unit 14 at a timing synchronized with the reference frequency signal supplied by the reference signal generator 16 . The required capacitance value is determined by integrating, averaging, or more generally filtering the individual measurements made by unit 14 . In this way, interference in the measurement is substantially excluded except for spurious signals with a strong frequency spectrum around the reference frequency.

Reference frequency generator 16 attempts to automatically select and generate a reference frequency which excludes the frequencies of the most undesirable spurious signals. This approach essentially eliminates interference, although its frequency may initially be unknown and may even change during operation.

The position locator 18 processes the capacitance measurement signal from the synchronous electrode capacitance measurement unit 14 and provides a position signal for use, eg, by a host computer, and supplies the position signal to the reference frequency generator 16 . The position locator unit 18 determines the finger position signal from the capacitive measurements. There are several different systems known in the art for determining finger position from measurements of capacitance associated with electrodes in an array. Position locators can provide one-dimensional sensing (such as slider control of variable resistors), two-dimensional sensing with contact determination (such as computer cursor control), or full three-dimensional sensing (such as for games and three-dimensional computer drawing). Examples of prior art position locators are shown at 40 and 50 in FIG. 1 of the aforementioned Gerpheide '017 patent.

Electrode array:

Figure 2A shows the electrodes in the preferred electrode array 12 and the coordinate axes defining the X, Y directions. This embodiment includes 16 X electrodes and 12 Y electrodes, but only 6 X electrodes 20 and 4 Y electrodes 22 are shown for clarity. Obviously, a person skilled in the art knows how to increase the number of electrodes. The array is preferably fabricated in the form of a multilayer printed circuit board 24 . These electrodes are etched conductive strips which are connected to vias 26 which in turn connect them to other layers in the array. For example, array 12 is about 65 millimeters in the X direction and about 49 millimeters in the Y direction. The width of the X electrodes is about 0.7mm, and the distance between centers is about 3.3mm. The Y electrodes are approximately 3 mm wide and spaced 3.3 mm center to center.

FIG. 2 b is a side cross-sectional view of the electrode array 12 . The insulating skin 21 is a layer of clear polycarbonate approximately 0.2 mm thick with a textured texture on the upper surface, the abrasion resistance of which can be increased by adding a textured clear hard coat to the upper surface. The underside of the skin can be screen printed with ink to provide patterns and colours.

The X electrodes 20, Y electrodes 22, ground plane 25 and component lines 27 are copper lines etched in a multilayer printed circuit board. The ground plane 25 covers the entire array area and shields the electrodes from electrical interference that may be generated by the circuit portion. Component lines 27 connect channels 26 and thus electrodes 20 , 22 to other circuit components of FIG. 1 . Insulation layers 31, 32 and 33 are fiberglass epoxy layers within the printed circuit board. Their thicknesses are approximately 1.0 mm, 0.2 mm, and 0.1 mm, respectively. These dimensions can vary as long as they remain consistent. However, all X electrodes 20 and all Y electrodes 22 must each have the same size.

Those skilled in the art will appreciate that different techniques and materials can be used to form the electrode array. For example, Fig. 3A shows another embodiment in which the electrode array includes an insulating skin 40 as described above. Conductive ink layers 42 and insulating ink layers 43 are alternately provided on the opposite side of the insulating surface layer 40 by a screen printing process. The X electrode 45 is located between the insulating surface layer 40 and the X electrode conductive ink layer 42 . The Y electrode 46 is located between the insulating ink layer 43 and the conductive ink layer 44 . Another insulating ink layer 47 is added below the conductive ink layer 44 and a ground plane 48 is added over the Y electrode conductive ink 47 . Each layer is approximately 0.01 mm thick.

The resulting arrays are thin and flexible so that curved surfaces can be formed. When in use, it should be placed on a strong solid support. In another example, the electrode array may utilize a flexible printed circuit board, such as a flexible printed wiring board or stamped metal sheets and foils.

There are various geometries and arrangements of electrodes that can be used for finger position detection. An example of this is shown in Figure 3b. This is an array 50 of parallel electrode strips for one-dimensional position detection, which can be used as a "slide potentiometer control" or "toaster darkness control". Other examples include control of a grid of diamonds or control of sec-tors of a disk.

It is expected that the electrode arrays of the present invention can be fabricated using inexpensive and widely available printed circuit board manufacturing processes. It is also hoped that a variety of surface materials can be used, and selected according to the texture structure and small friction force, on which it should be possible to print artistic calligraphy and colors economically. It is also contemplated that the skin layer can be printed separately from the electrode-containing portion of the array. This allows the mass production of parts containing electrodes, which are then fixed with specially printed skins.

Synchronous Electrode Capacitance Measurement Unit:

Figure 4 shows a more detailed view of the synchronization electrode capacitance measurement unit. Its main parts include: (a) a part for generating voltage changes in the electrode array synchronously with a reference signal, (b) a part for generating a signal representing the displaced charge and thus also the state of coupling between the electrodes of the electrode array, (c) a section for detecting the signal synchronized with the reference signal, and (d) a section for low-pass filtering the detected signal. Unit 14 is preferably connected to the electrode array via a multiplexer or switch. The capacitance to be measured in this embodiment is the mutual capacitance between the electrodes, but may also be the stray capacitance of the electrodes to ground or the algebraic sum (ie sum or difference) of said mutual or stray capacitances.

Figure 4 shows a specific example of a synchronous electrode capacitance measuring unit 14 connected to the electrode array 12 for measuring the algebraic sum of the mutual capacitances between the electrodes. All components are divided into 4 main functional blocks. Active electrode synchronization block 70 connects each X electrode to one of lines CP or CN, and each Y electrode to one of lines RP or RN. The electrodes are selectively connected to these lines by switches, preferably CMOS switches under the control of a position locator device 18 (FIG. 1), for selecting the appropriate electrode for capacitance measurement. (See the '017 patent to Gerpheide, which illustrates this electrode selection by signal S in FIG. 1.) All electrodes connected to the CP line at any one time are considered to form a "positive effective X electrode". Similarly, the electrodes connected to CN, RP, and RN form "negative effective X electrode", "positive effective Y electrode" and "negative effective Y electrode", respectively.

The reference frequency signal is preferably a digital logic signal from reference frequency generator 16 (FIG. 1). The reference frequency signal is supplied to the unit 14 through an AND gate 72 which also has a "drive enable" input which is signaled by the reference frequency generator 16 (FIG. 1). The output of the AND gate is fed through an inverter 74 and a non-inverting buffer 76 into lines RP and RN, respectively, which are part of a capacitance measurement section 78. In the preferred embodiment, the drive enable signal is always TRUE to pass the reference frequency signal. In another preferred embodiment, it is set to FALSE by the reference frequency generator when the interference calculation is to be performed as described later. When the drive enable signal is FALSE, the drive signal stays at a constant voltage level. When the drive signal is TRUE, it is a replica of the reference signal.

Capacitance measurement section 78 includes a differential charge transfer circuit 80 as shown in Figure 4 of Gerpheide US Patent 5,349,303, incorporated herein by reference. Capacitors CS1 and CS2 of FIG. 4 of the aforementioned patent correspond to the stray capacitances of the positive and negative active electrodes to ground. The CHOP signal of FIG. 4 is a general square wave signal in the present invention having a frequency equal to half of the reference frequency signal and generated by divide-by-two circuit 81 as shown. As described in the '303 patent to Gerpheide, this circuit maintains CP and CN (lines 68 and 72 therein) at a constant virtual ground potential.

Capacitance measurement section 78 also has a non-inverting drive buffer 76 that drives the RN and negative active Y electrodes to vary voltage levels such that changes in the drive enable signal are replicated. The inverting buffer 74 drives the RP and positive active Y electrodes to change voltage levels inversely to the change of the drive enable signal. Because CP and CN are held at virtual ground, these voltage changes are the net voltage changes across the mutual capacitance that exists between the effective Y electrode and the effective X electrode. A charge proportional to the change in these voltages multiplied by an appropriate capacitance value is thus coupled to nodes CP and CN ("coupled charge"). Thus, the charge transfer circuit provides a proportional differential charge at its outputs _Q01 and _Q02 that is proportional to the coupled charge and thus to the capacitance.

Briefly, this differential charge is the product of the proportionality factor K times the "balance" L, where L is the combination of these capacitances given by:

L=M(xp, yn)+M(xnyp)-M(xp, yp)-M(xn, yn) where M represents the mutual capacitance between the effective electrode "a" and the effective electrode "b". The charge at equilibrium represents the finger position relative to the effective electrode position as described in US Pat. No. 5,305,017 to Gerpheide. The proportional system K has the same sign as the direction of change of the drive enable signal.

Referring again to FIG. 4 , the synchronous electrode capacitance measurement section 78 is connected to a synchronous detector 82 through lines with charges Q ₀₁ and Q ₀₂ , which may be a double-pole double-throw CMOS switch 84 controlled by a reference frequency signal. The synchronous detector 82, whose main function is to rectify the charges _Q01 and _Q02 , is connected to a low pass filter 86 which may be a pair of capacitors C1 and C2 forming an integrator for the differential charge. (For example the integrator could be a low pass filter with 6db per octave attenuation.) When the reference frequency signal just goes positive and K is positive, the charges Q ₀₁ and Q ₀₂ are integrated on capacitors C1 and C2 respectively. When K is negative, the charge is integrated on the opposite capacitor. In this way, a differential charge proportional to the balance L is accumulated on capacitors C1 and C2.

FIG. 5 is another embodiment of the synchronous electrode capacitance measurement unit 14 . In this embodiment, each electrode in the electrode array 90 is connected to a dedicated capacitance measurement section 92 , each dedicated capacitance measurement section 92 is connected to a synchronous detector 94 and then to a low pass filter 96 . The advantage of this configuration is that it can continuously provide capacitance measurements for each electrode, whereas the previous preferred embodiment can only measure one electrode configuration at any one time. The disadvantage of the embodiment of FIG. 5 is the greater outlay, which may be related to repeated elements. This is a common exchange between providing multiple elements to take measurements simultaneously and multiplexing one element to take measurements sequentially. Obviously, this exchange can be applied in a large number of different fields.

Furthermore, many units can be implemented in digital form using analog-to-digital converters and digital signal processing. While the preferred embodiment currently uses basic analog processing, it is anticipated that digital processing may be relatively inexpensive in the future.

FIG. 6 provides several preferred alternatives to capacitance measuring portion 78 (FIG. 4) or 92 (FIG. 5). The circuits shown in Figures 6A and 6B are suitable for measuring the mutual capacitance between electrodes (which may be actual electrodes or virtual electrodes), denoted by Cmp, Cmn, and Cm. The circuits shown in Figures 6C and 6D are suitable for measuring the capacitance of an electrode to ground, denoted by Cg. Each of these circuits provides an output voltage change representative of the capacitance being measured. These voltage changes are given by:

For Figure 6A: ΔV _output = ΔV _drive × (Cmp-Cmn)/Cr

For Figure 6B: ΔV _output = ΔV _drive × Cm/Cr

For Figure 6C: ΔV _output = ΔV _drive × Cg/(Cg+Cr)

For Figure 6D: ΔV _output = ΔV _drive × (Cg+Cr)/Cg

These equations relate to a known reference capacitance represented by Cr and a known change in drive voltage represented by _ΔVDrive . Other capacitance measurement sections can be based on charge balancing techniques, as described in Meyer, US Patent 3,857,092. Synchronous detectors can be implemented using analog or digital amplifiers, or using a National Semiconductor Company "double-balanced mixer" integrated circuit (eg part number LM1496). Regarding the low-pass filter section, there are many different implementations in the prior art, such as switched capacitor integrators and switched capacitor filters, high order analog filters and digital filters.

Reference Frequency Generator:

Figure 7 is a preferred embodiment of the reference frequency generator (Figure 1). The generator observes the position signal in order to estimate the degree of interference at a certain interfering frequency. In the case of detection of substantial interference, the generator 16 then selects a different frequency for further measurements. Generator 16 always attempts to select a frequency for measurement that is different from that which has been found to cause measurement disturbances, as described below.

The generator 16 includes an oscillator 100 , whose oscillation frequency is set to 4 MHz, for example, for driving a microcontroller 102 and a divide-by (M+N) circuit 104 . where the value N is a fixed constant around 50. The value of M is determined by the microcontroller 102 , for example, may be one of four values specified by the microcontroller 102 in the range of 61 KHz to 80 KHz.

Microcontroller 102 performs the functions of interference estimation 106 and frequency selection 108 . It can complete other functions related to the system, such as location positioning. The preferred interference estimation function 106 generates a measure of the interference in the position signal generated by means of the position location unit 18 (FIG. 1). It is based on the principle that interference produces a spurious high frequency component of the position signal and operates as described in the code below. Assume that position data points X, Y, and Z occur approximately every 10 milliseconds. Briefly, it computes, for 32 data points, the sum of the absolute values of the quadratic differences of X and Y and the absolute value of the first difference of Z as the disturbance measure IM. Computing the difference of a data stream has the effect of high-pass filtering the data stream.

In detail, for each data point, the disturbance estimation function 106 performs the following steps, where ABS() denotes the absolute value function:

XD=X-XLAST; calculate a difference

YD=Y-YLAST

ZD=Z-ZLAST

XDD=XD-XDLAST; calculate the second difference;

YDD=YD-YLAST

IM=IM+ABS(XDD)+ABS(YDD)+ABS(Z); sum

If 32 samples per section

{execute frequency selection function 108 (see the following description)

IM=0}

XLAST=X; move the current value to the nearest value

XLAST=Y

ZLAST=Z

XDLAST = XD

YDLAST=YD

In another embodiment, instead of estimating based on the position signal, the disturbance estimation function 106 asserts the aforementioned drive enable signal to a FALSE state and reads out the resulting synchronous capacitance measurement. This measures the charge coupled to the electrodes when no voltage is applied across them by the device. These charges must be the result of a disturbance, so this disturbance (from the spurious signal) is measured directly. This is another way to generate an interfering measurement IM.

The preferred frequency selection function 108 generates a list of historical interference measurements for each frequency that may be selected. At system initialization, each item is set to zero. Thereafter, the frequency selection function is activated by the interference estimation function 106 approximately every 32 data points. The current interference measure IM for the currently selected frequency is entered as an entry in the table. Then scan all entries. The frequency with the lowest interference metric term is selected as the new current frequency, and the corresponding M value is sent to the divide by (M+N) unit 104 . Each entry in the table is decremented approximately every 80 seconds by an amount corresponding to a position change of approximately 0.05mm. In this way, if a frequency is marked as "bad" because it had strong interference once, it will not be permanently marked as "bad".

For different embodiments, the above-mentioned functions can be realized by a microprocessor, for example, a microprocessor manufactured by Motorola with the serial number MC68HC705P6 as the microcontroller 102.

FIG. 8 shows another preferred embodiment of the reference frequency generator 16 (FIG. 1). It generates a randomly varying reference frequency signal. Each cycle of the signal has a different and essentially random time interval. It is extremely unlikely that spurious signals will consistently follow the same sequence of random changes. Thus, spurious signals are substantially rejected by capacitance measurements synchronized with the reference frequency. Although not as extensive as the previous embodiment, this generator is simpler since no interference estimation and frequency selection functions are required.

The generator of FIG. 8 includes an oscillator 110 and a divide by (N+M) circuit 112 . The value M provided to the divider comes from a pseudo-random number generator (PRNG) 114 which generates numbers in the range 0-15. Each cycle of the reference frequency controls the PRNG 114 to generate a new number synchronously therewith. PRNGs are well known in the art.

For each of the embodiments in Figure 7 or Figure 8, the range of values of M relative to the value of N can be increased or decreased to give a larger or smaller range of possible frequencies. The value of N or the oscillator frequency can be adjusted, thereby changing the maximum possible frequency. Instead of a divide by (M+N) circuit, a phase-locked frequency synthesizer, such as Motorola MC145151-2, or a voltage-controlled oscillator driven by a D/A converter may also preferably be used.

It should be understood that other variations of the preferred embodiments described above fall within the scope of the present invention. These variations include different electrode array geometries such as grids of bars, grids of diamonds, parallel bars, and various other shapes. Various modifications of the method of fabricating the electrode array are also contemplated. Examples include printed circuit boards (PCBs), flex PCBs, screen printed, stamped metal sheets or foils. Also included are variations in the type of capacitance used, such as fully balanced (see Gerpheide's '017 patent), stray, mutual, half balanced.

The foregoing description has provided certain preferred embodiments according to the invention. Obviously, various modifications can be made by a person skilled in the art within the scope of the appended claims, and the following claims include the inventive idea.

Claims (21)

1. A capacitive-based proximity detector for determining the position of an object while rejecting electrical interference, comprising: (a) an array of electrodes for forming a capacitance that varies with the motion of the object; (b) a measuring device connected to said electrode array for measuring said capacitance synchronously with a reference signal; and (c) Generator means for supplying the measuring means with a reference signal having a frequency incoherent with the frequency of the electrical interference signal. 2. A proximity detector as claimed in claim 1, characterized in that the generator means comprise means for estimating electrical disturbances and for generating a reference signal, the frequency of said reference signal being different from the frequency of said electrical disturbances. 3. Proximity detector as claimed in claim 2, characterized in that the evaluation means comprises a table for storing the frequencies of the selected reference signals and the measured value IM of the electrical interference for each said frequency, and for generating the reference signals means that the frequency of the reference signal has the lowest IM associated therewith. 4. A proximity detector as claimed in claim 1, characterized in that the generator means comprises oscillator means for generating a reference signal and means for randomly varying the frequency of the reference signal generated by said oscillator means. 5. Proximity sensor as claimed in claim 4, characterized in that said changing means comprises means for changing the frequency of the reference signal in proportion to a supplied number, and means for continuously supplying the number to the changing means Pseudo-random number generator. 6. The proximity detector of claim 1, further comprising object positioning means for generating a position signal having a high frequency component representing the position of the object relative to the electrode array in response to the measuring means, and wherein said occurrence Devices include estimating means; for determining the magnitude of the high frequency component of the position signal in response to the object locating means, and means for varying the frequency of the reference signal in response to the estimating means when the magnitude of the high frequency component of the position signal exceeds a predetermined value. 7. The proximity detector of claim 1, wherein said measuring means comprises driver means for generating a voltage change across the electrode array synchronously with the reference signal, a charge measuring device for measuring the charge coupled to the electrode array, and thus the capacitance, synchronously with the reference signal, coupled charge measuring means for selectively disabling the drive means from producing a voltage change, and measuring a measure IM representative of the disturbance during the disabling of the drive means, and Wherein said generator means comprise means for changing the frequency of the reference signal when the interference measure IM exceeds a predetermined value. 8. Proximity detector as claimed in claim 7, characterized in that the generator means further comprises a table for storing the frequency of the reference signal and the related interference measure IM measured for the reference signal of each frequency, and for generating the reference means of the signal, the frequency of the reference signal having the lowest interference measure IM associated therewith. 9. The proximity detector of claim 1, wherein the electrode array comprises a plurality of first electrode strips spaced apart from each other in a first array, and a plurality of second electrode strips spaced apart adjacent to the first electrode strips . 10. A proximity detector as claimed in claim 9, wherein the measuring means comprises: driving means for generating voltage changes on the electrode array synchronously with the reference signal, charge transfer means, connected to the electrode array, for generating a measurement signal representative of the charge coupled to the electrode array due to the voltage change, synchronously with the frequency of the reference signal, synchronous detection means, connected to the charge transfer means, for rectifying the measurement signal generated synchronously with the reference signal, and Low-pass filter means, connected to the synchronous detection means, are used to generate a signal representative of the average DC voltage of the rectified signal and thus also of the capacitance of the electrode array. 11. A proximity detector as claimed in claim 10, wherein said measuring means comprises a plurality of capacitive measuring sections, each connected to a respective electrode strip. 12. The proximity detector of claim 11, further comprising a plurality of synchronous detection sections, each connected to a respective capacitance measurement section. 13. The proximity detector of claim 1, further comprising position locating means coupled to the output of the measuring means for generating a position signal representative of the position of the object relative to the electrode array. 14. A capacitive-based touch detection system for eliminating interference and generating data by detecting the position of an object relative to a touchpad, the system comprising: (a) an array of electrodes adjacent to the touch pad for generating capacitance, (b) means for measuring, synchronously with the reference frequency signal, a change in capacitance adjacent the touch pad as an object moves relative to the pad, and (c) means for generating a reference frequency signal supplied to the measuring device, said reference frequency signal having a frequency remote from the frequency of the disturbance so as to effectively reject said disturbance. 15. A detection system as claimed in claim 14, wherein the reference frequency generator means comprises means for estimating the frequency of the disturbance and for selecting a reference frequency signal whose frequency is away from the frequency of the disturbance. 16. The detection system as claimed in claim 14, wherein the reference frequency generator means comprises an oscillator and a pseudo-random number generator connected to the oscillator for generating the reference frequency signal whose frequency is random. 17. The detection system of claim 14, wherein the electrode array includes first and second separate sets of electrodes for generating capacitance to the touch pad. 18. The detection system of claim 17, wherein the first and second electrode sets are substantially perpendicular to each other so as to form a virtual elec-trode for providing capacitance. 19. The detection system as claimed in claim 14, wherein said measurement device comprises a capacitance measurement part connected to the electrode array, a synchronous detector connected to the capacitance measurement part, and a low pass filter connected to the detector. 20. A method for detecting the position of an object on an electrode array comprising the steps of: (a) producing a capacitance across said array that varies with the motion of the object, (b) measuring the capacitance across the array synchronously with the frequency of the reference signal, and (c) generating a reference signal having a frequency incoherent with the frequency of the electrical disturbance affecting said capacitance. 21. The method of claim 20, further comprising the step of generating a signal representative of the position of the object relative to the electrode array.

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